1. Field of the Invention
This invention relates to a thermal image transfer recording medium which can yield images with high density, with a minimum decrease in the image density even when it is repeatedly used.
2. Discussion of Background
Recording apparatus, such as a printer and a facsimile apparatus, to which the thermal image transfer recording method is applied, are now widely used. This is because the recording apparatus of this type are relatively small in size and can be produced inexpensively, and their maintenance is simple.
In a conventional thermal image transfer recording medium for use with a thermal image transfer recording apparatus, a single ink layer is formed on a support. When such a recording medium is used for printing images, a heated portion of the ink layer, for instance, by a thermal head, is completely transferred to an image receiving sheet by one-time printing only. Therefore, the recording medium can be used only once, and can never be used repeatedly. The conventional recording medium is thus disadvantageous from the economical point of view.
In order to overcome the above drawback in the prior art, there have been proposed the following methods:
(1) A microporous ink layer is formed on a support so that a thermofusible ink impregnated in the ink layer can gradually ooze out, as disclosed in Japanese Laid-Open Patent Applications 54-68253 and 55-105579;
(2) A porous film is provided on an ink layer formed on a support so that the amount of ink which oozes out from the ink layer can be controlled, as disclosed in Japanese Laid-Open Patent Application 58-212993; and
(3) A plurality of ink layers are formed on a support, with a plurality of adhesive layers interposed between the ink layers. These ink layers can be exfoliated one by one while images are printed, as disclosed in Japanese Laid-Open Patent Applications 60-127191 and 60-127192.
However, the above three methods have the shortcomings as described below.
When the above method (1) is employed, the ink cannot sufficiently ooze out after the repeated use of the recording medium. As a result, the density of printed images gradually decreases as the number of printing times increases.
As to the method (2), the mechanical strength of the porous film is decreased when the size of the pore included therein is increased in order to increase the image density, and thus the ink layer is apt to peel off the support together with the porous film.
As to the method (3), the amount of the thermofusible ink which is transferred to an image-receiving sheet cannot be uniformly controlled when images are printed.
Furthermore, most of the conventional methods have been developed for use with a serial thermal head for a recording apparatus such as a word processor. Therefore, when those methods are applied to a line thermal head for use in a recording apparatus such as a facsimile apparatus or a bar code printer, problems are brought about, for instance, exfoliation of an ink layer, and decrease in the image density when the recording medium is repeatedly used.